Dairy Pipeline: February 2004
Extension, Dairy Scientist,
Genetics and Management
Your monthly DHI 202 lists projected 305 day, 2X, ME averages for first calf heifers, cows in second lactation, and cows in third or later parities. The youngest cows in the herd should be genetically superior because they result from more recent sire selection decisions. Older cows are survivors of culling decisions for milk yield. So which group should give the most milk? The table below shows the average age of first, second, and third and later lactation cows for alternate years over the past decade, along with the differences in 305d-2X-ME milk yields.
Table 1. Trends in age and ME milk yield for Virginia cows on supervised DHI plans.
|Date||Average age (mo)||Difference in 305d-2X-ME milk yield (lbs)|
|First lactation||Second lactation||Third+ lactation||Second minus first||Third+ minus Second|
Heifers don't give as much milk as second lactation cows, so the effect of culling offsets the genetic advantage of the younger cows. Notice that the trend is towards younger cows in all three groups. Heifers freshen younger, but older cows are younger, too. Herd size in Virginia has increased from 124 cows/herd in December 1995 to 137 cows/herd today. Older cows are at a disadvantage in many larger herds because of time spent on concrete, in the holding pens, on the way to and from the parlor. And there is little patience for the needs of older cows as herd size increases, unless management is willing to accommodate the older girls. The difference in ME's between second and third parity cows suggests a trend to harder times for the older cow in Virginia herds in recent years. The trend is somewhat disturbing. Are we becoming so concerned with efficiency of moving cows through the parlor that we don't give the older cow a chance, or is the old girl simply more trouble than she's worth? I'm at the age where biased towards the elderly takes over, but there is reason to question the trends towards less milk in older cows compared to second lactation cows. Older cows do carry a lifetime's accumulation of war wounds, perhaps scar tissue from mastitis and mobility problems. They seem reasonably fit, reproductively speaking, averaging 163 days open across the state compared to 161 days for second lactation cows. But they don't milk like the younger cows. Check your management system for evidence of discrimination against the elderly in your operation. It makes economic sense to give your senior citizens every reasonable opportunity to make you money, and the trend in the table says that we don't do as good a job of that as we used to do.